Bistable electro-optical network



July 3, 1962 J. F. VIZE BISTABLE ELECTRO-OPTICAL NETWORK Original Filed Dec. 2'7, 1957 & m2 W WF 5 m w 'IIIIIIIIIIIIIIIIIIIIII ATTORNEY 3,042,807 BISTABLE ELECTRO-OPTICAL NETWORK James F. Vize, Rhinebeck, N.Y., assignor to General Electric Company, a corporation of New York Original appiication Dec. 27, 1957, Ser. No. 705,680, now Patent No. 2,997,596, dated Aug. 22, 1961. Divided and this application Jan. 19, 1961, Ser. No. 83,731 8 Claims. (Q1. 250-213) This invention relates to bistable electro-optical networks, and more particularly to electrical networks including electroluminescent phosphors and photoconductors as elements thereof and adapted to operate in either one of two stable states.

The phenomenon of electroluminescence upon which the operation of the networks of the present invention in part depends is the process by which certain semiconducting materials, known as phosphors, emit radiation under the primary stimulus of an applied electrical field or potential. For a survey and bibliography on the subject of electroluminescence, reference is made to an article by Destriau, G. and Ivey, H. F., Electroluminescene and Related Topics, Proceedings of the Institute of Radio Engineers, vol. 43 (1955), pp. 1911-1940.

As noted in the above article electroluminescent phosphors have in the past been used as light sources in devices frequently called electroluminescent capacitors or electroluminescent cells. Such devices often resemble a fiat plate capacitor and may comprise two parallel planar electrodes which have sandwiched between them, in one form or another, an electroluminescent phosphor. The phosphor may be in the form of microcrystals suspended in a transparent plastic or dielectric binder. Alternatively, the phosphor may be in the form of a continuous, transparent crystalline layer such as that disclosed in U.S. Patent No. 2,709,765 to L. R. Koller, or in the form of single crystals as disclosed in U.S. Patent No. 2,721,- 950 to Piper and Johnson. In general the microcrystalin-plastic type of phosphor dielectric exhibits electroluminescence only under excitation by alternating electric fields, whereas in the two patents referred to above, the phosphors exhibit electroluminescence when excited by either alternating or unidirectional electric fields. The carrier-injection electroluminescence described in the aforementioned article is a type of electroluminescence excited by unidirectional electric fields.

Prior known electro-optical networks employing both electroluminescent phosphors and photoconductors disposed and interconnected for mutual cooperation have been used as amplifiers or oscillators, etc. Networks of this type are shown in US. patent application Serial No. 585,027 by R. E. Halsted and J. F. Elliott and U.S. patent application Serial No. 585,052 by C. F. Spitzer, both applications being assigned to the assignee of the instant application.

The term photoconductor as used herein is intended to apply to any material the impedance or conductivity of which varies as a function of the radiation emitted by a particular associated electroluminescent phosphor. A photoconductor is said to be in radiation-coupled relationship with an electroluminescent phosphor when they are so related that the impedance or conductivity of the photoconductor varies as a function of the radiation emission of the electroluminescent phosphor. Further, the network in which the photoconductor and the electroluminescent phosphor are included will be said to be an electro-optical networ whenthere is an interaction in the network between electrical energy and radiant or light energy.

An electroluminescent cell and a photoconductor connected in electrical series relation and positioned in radiation-coupled relationship may be termed an electrotense: Fatented July 3, 1962 optical pair, for purposes of this specification. Such an electro-optical pair, when connected across a predetermined value of voltage is adapted to be bistable; that is, the pair will draw only one of two possible values of current, one high and one low. Correspondingly, the

and is adapted to be shifted from either one of its stable states to the other by application of a radiation trigger signal to the respective photoconductor coupled to the dark electroluminescent cell. Such a bistable apparatus is extremely useful, especially in digital computers for register and counter elements. When used as a register element, the two stable states would be designated respectively as the binary digits 1 and 0. Input radiationtriggering means is provided to independently switch the device to either of its stable states. When used as a counter element, the device is switched from the stable state in which it is operating to its other stable state upon application of a common input trigger signal; that is, the device would return to a particular stable state after each two input trigger signals.

A bistable device employing these electroluminescent phosphors has the additional desirable feature of providing a visible indication of the state of the device. Thus, one electrolumniescent phosphor glows only when the network is in one of its two stable states and the other electroluminescent phosphor'glows only when the network is in the other stable state.

It is therefore a principal object of this invention to provide a bistable device comprising two mutually interrelated electro-optical pairs.

Another object of this invention is to provide a-novel bistable electro-optical network.

Another object of thisinvention is to provide an electrical network including electroluminescent phosphors and photoconductors as elements thereof and adapted to operate in either one of two stable states.

Another object of this invention is to produce an output signal from an electro-optical network in response to every two" input signals.

Another object of this invention is to provide an electrical network including electroluminescent phosphors and photoconductors as elements thereof and adapted to operate in either one of two stable states, wherein the particular phosphors luminescing are indicative of the stable state in which the network is operating.

The foregoing objects are achieved by providing networks having first and second electro-optical pairs connected in parallel. A source of electrical energy is coupled across the parallel-connected electro-optical pairs. A point of the first electro-optical pair between the electroluminescent cell and the photoconductor thereof is connected to a point of the second electro-optical pair between the electroluminescent cell and the photoconductor thereof. This connection between the first and second electro-optical pairs is one means whereby feedback is provided so that the network can operate in only one of two stable states; that is, wherein the on electro-optical pair draws relatively large current and its electroluminescent phosphor emits a relatively intense radiant energy signal, and the off electro-optical pair draws relatively little current and its electroluminescent phosphor emits relatively little, if any, radiant energy. Upon application a it e,ea 2,so7

of a radiant energy trigger signal to the photoconductor of the o electro-optical pair the network shifts to its other stable state. In this other stable state the formerly 01f electro-optical pair is on and vice versa.

Theinventionwill be described with reference 'to the accompanying drawings, wherein:

FIGUREI is a circuit diagram of the bistable network I of this invention, including one form of triggering means;

7 broken linebetween them. The arrows and broken lines elsewhere in'FIGS. l and 2 indicate the same radiation- 7 coupled relationship and this convention will be employed throughout this application. Electro-optical pair 12 com- I prises a series-connected electroluminescent cell 15 and photoconductor 16 positioned in radiation-coupled relationship. A lead 17-interconnects points '18 and 19 located between the electroluminescent cell and-photoconductor of pairs 11 and 12, to provide the feedback, 'described hereinafter, for causing this network to operate in only one of two stable states; 7

Examples of electro-optical pairs useful in the circuit of FIG. 1 are shown and described in the aforementioned U.S. patent application Serial No. 585,052 by C. F. Spitzer. A device 31'includingfsuch an electro-optical pair is shown in FIG. 3. Device 31 is adapted to receive either or both electrical and light input signals and to produce either or both electrical and light output signals. In device 31 an electrode 32 consists of a rigid opaque metallic member serving as bothan electrode and a supporting member and which is preferably polished for maximum lightreflection. Deposited on one side of electrode 32 are an electroluminescent layer 33, a lighttransmitting electrode 34-, a photoconductive layer 35, and alighttransmitting electrode 36. Deposited, on the other side a of electrode 32 is an electroluminescent layer 43, a lighttransmitt ing electrode 44, a photoconductive layer 46 and a light-transmitting electrode 47. 7 .Lead wires are soldered or otherwise electrically connected to each electrode.

Electroluminescent layer 33 and adjacent electrodes 32 and 34 comprise an electroluminescent cell EL-1, and electroluminescent layer. 43 and adjacent electrodes 32 and 44 comprise another electroluminescent cell EL,2;

'The above two electroluminescent cellsare connected'in' series by the common electrode 32 so as to make the cells EL-land EL-2 effectively one electroluminescent cell which is in radiation-coupled relation with two photoconduotors for reasons that will appear later. Similarly,

.photoconductive layer '35 and its electrodes 34 and 36 comprise one photoconductor PC1 and the photoconductive layer 46 and its, electrodes 44 and 47 comprise a second photoconductor PC-Z. A casing 48 which consists of a light-opaque, electrically-insulating material is used to support the cells, photoconductors and a lens 49 adjacent electrode 36. It should be understood that the word light, as used inthisapplication, includes any radiation emitted by an electroluminescent phosphor to which a photoconductor is responsive and may, for example, include ultraviolet or infrared radiation. 7

Light-transmittng, electrical conducting electrodes 34 and 36 may be layers of titanium dioxide or tin oxide, commonly referred to as conducting glass. Alternatively, a very thin light-transmitting layer of evaporated metal, such as aluminum or silver, may be used. If the lighttransmitting, electrical conducting electrodes are titanium dioxide they may be prepared and rendered. conductive I in accordance with the teachings of U.S. Patent No.

2,717,844to L. R. Koller.

Electroluminescent layers 33 and 43 may be phosphors such as zinc sulfide activated by three-tenths percent by weight of copper and written as ZnSzCu, prepared as a continuous crystalline layer, as disclosed. in the abovementioned Patent 2,709,765 to L. R. Koller, or single crystalline phosphors of the type disclosed in the abovementioned Patent 2,721,950 to Piper and Johnson. These types of electroluminescent layers are responsive to both direct or alternating electric fields. The average brightness B of the light output of an electroluminescent phos- 'phor as a function of the voltage applied to it may be closely approximated by the expression,

. where n' is a constant characteristic of the particular electrolurninescent phosphor used and -k is a'co'nstant of proportionality. Values of n, in Equation 1, range approxiinately from 1 to 7 for known phosphors.

Photoconductive layers 35 and 46 are thin light-permeable layers of photoconductive material. This material may, for example, comprise cadmium sulfide or lead sultide, which may be sprayed, sputtered, or evaporated on one of the light-transmitting electrodes 34 or 36. and 44 or 47. More generally, photoconductive layers 35 and 46 may, for example, consist of any of the sulfides, selenides, or tellurides of cadmium, lead, or zinc, or may be any other known photoconductor.

The physical arrangement of the devices 31' is such that light emitted by'electroluminescent cell EL-1 falls on photoconductor PC-l. This cell and photoconductor are connected in electrical series relation between terminals {Wand 42, and therefore constitute an electrooptical pair such as pair '11 or 12 (FIG. 1). The current drawn by this pair depends on the voltage applied to the PC-1 and PC2. The relationship between the diagrammaticillustrationof FIG. 1, for example, and the physical embodiment of FIG. 3 may be understood, by noting that there is one device 31 for each of the electro-optical pairs 11 and :12 (FIG. 1 and their radiation-coupled photoconductors 25 and 28, respectively. Light may be transmitted from electroluminescent'cell 27 to photoconductor 14 (FIG. 1) through lens 49 (FIG. 3). if it is desirable to physically separate the cell and .photoconductor, or alternatively,'the lens may be eliminated by locating electroluminescent cell 27 (FIG. 1) adjacent photoconductor PC-1 (FIG. 3 with the electrode 36 serving as a common electrode for the two. The device 31 for the electrooptical pair 12 may be similarly arranged for receiving light from electroluminescent cell 24. a

' Referring once more to FIG. 1, a source 21 of electrical energy is connected across the parallel-connected V relatively large current and electroluminescent cell 13 emits a relatively intense light signal, as compared with the light emitted by cell "15 of electro-optical pair 12. Each of photoconductors "14 and 16 have an impedance which decreases as the intensity of light falling thereon increases. Each of cells 13 and 15 emits a light signal, the intensity of which increases as the voltage applied to the cell increases. Thus, electroluminescent cell 13 illuminates photoconductor .14 causing its resistance to be relatively low compared with that of photoconductor 16, which has little if any light falling upon it. Since photoconductor '14 has a relatively low resistance a low voltage is coupled through lead 17 to electroluminescent cell 15 to maintain it substantially dark. Photoconductor 16 is thus maintained unilluminated with relatively high resistance, preventing electroluminescent cell 15 from lighting. This condition is designated the first of the two stable states of the network. In the designated second stable state of this network electro-optical pair 12 is onf and electro-optical pair 11 is maintained ofi by feedback through lead 17 in a manner similar to that described with reference to the above-mentioned first stable state.

The network of FIG. 1 when operating in one of its two stable states is adapted to be switched to its other stable state upon application of a light signal or an electrical signal to the off electrooptical pair. Consider again the above-assumed first stable state operation in which electro-optical pair '12 is off. Upon application of a light signal to photoconductor .16 from a source external to electro-optical pair 12 the resistance of photoconductor 16 decreases. As a result of this resistance decrease the voltage which is applied to electroluminescent cell 15 increases. This change in electrical condition of elect-ro-optical pair 12 is coupled through lead 17 and tends to reduce the voltage across electroluminescent cell 13, which thereupon decreases its light output and correspondingly increases the resistance of photoconductor '14. It the external source of radiation applied to photoconductor 16 is maintained for a sufficient duration the current drawn by electro-optical pair 12 continues to increase toward its stable on value and electro-optical pair 11 switches to the off condition. A detailed analysis of the operation and switching of this bistable network is provided later.

Electrical circuit branches 22 and 23 connected in parallel across electro-optical pairs 11 and 12 provide one means for triggering the bistable network from one state to, the other. Branch 22 comprises an electroluminescent cell 2 a photoconductor 25, and a photoconductor 26 connected in series. Electroluminescent cell 24 is positioned in radiation-coupled relationship with photoconductor 16. Photoconductor 25 is positioned in radiationcoupled relationship with electroluminescent cell 13. Branch 23 comprises an electroluminescent cell 27, a photoconductor 28, and a photoconductor 29 connected in series. Electroluminescent cell 27 is positioned in radiation-coupled relationship with photoconductor 14. Photoconductor 23 is positioned in radiation-coupled relationship with electroluminescent cell 15. Photoconductors 26 and 29 are adapted to receive light radiation signals fiom a common input trigger source 36, which may be, for example, another electroluminescent cell.

Assume, once again, that electro-optical pair 11 is on. In this condition, photoconductor 25 is illuminated by electroluminescent cell 13, but photoconductor 28 remains unilluminated since electroluminescent cell 15 is dark. In the absence of a radiation input trigger signal, photoconductors 26 and 29 have high values of resistance, and under this condition electroluminescent cells 24 and 27 are dark. Upon application of a radiation input trigger signal to photoconductor 26 its resistance is decreased and a large proportion of the voltage of source 21 is applied to electroluminescent cell 24. Electroluminescent cell 24 thereupon lights, illuminates photoconductor 16, and initiates the change of operational time requirement of photoconductor 25 exists because electroluminescent cell 24 should remain lighted until the change of state has been completed, despite the fact that electroluminescent cell 13 is growing dimmer. Likewise, photoconductor 23 must have a relatively slow rate I of change of impedance similar to that of photoconductor 25.

Application of the same or simultaneous radiation input trigger signal to photoconductor 29 when the bistable network is in its first stable state will, however, not affect electroluminescent cell 27 because of the high resistance of unilluminated photoconductor 28. Because of the slow rate of change of resistance of photoconductor 23 its impedance wil not become sufliciently low for electroluminescent cell 27 to light during the switching interval. Y

When the bistable network is in its second stable state the application of a radiation input trigger signal to photoconductor 29 will switch the bistable network to its first stable state in a manner similar to that describe above.

From the foregoing description it will be noted that in the embodiment shown in FIG. 1, the radiation input trigger signals are applied simultaneously from a common source 30 to photoconductors 26 and 29, and the circuit Will change its state of operation each time the trigger signals are applied. Thus, a particular state of operation of the network is repeated for every two input trigger signals, and the network acts as a counter element or an element for dividing by two a series of input trigger signals.

A useful output from the network may be derived as either an electrical or an optical signal from several points. For example, the voltage across, and the light output of, electroluminescent cell 13 are relatively large for every two input trigger signals, and either this voltage or light may be applied as an input signal to a similar succeeding bistable network or to any utilization device.

FZGURE 2 is a modified embodiment of the circuit diagram shown in FIG. 1 and includes photoconductor 52 which is common to the electrical circuit branches 22' and 23 employed for changing the state of the bistable network.

As before, one of photoconductors 25 and 28 is illuminated in accordance with the state of operation of the bistable network. Upon application of a radiation input trigger signal to photoconductor 52 from a trigger source 53, the electrical circuit branch whose photoconductor was illuminated will draw a relatively heavy current through photoconductor 52, and the electroluminescent cell of this heavily conducting branch will light and initiate the change of state in the manner previously described.

A theory of the operation of the bistable apparatus of this invention, as presently understood, and a method of determining certain circuit parameters are described in a copending application Serial No. 705,680, filed December 27, 1957, from which the present application has been divided.

While the principles of the invention have now been made clear in illustrative embodiments, there will be immediately obvious to those skilled in the art many modifications in structure, arrangement, proportions, the elements, materials, and components, used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operating requirements, without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

second photoconductor being positioned'in radiationcoupled relationship, the second electroluminescent cell and thefirstphotoconductor being positioned in radiationcoupled relationship, a third electroluminescent cell having one terminal connecte d to said third point, said third electroluminescent cell and said second photoconductor being positioned in radiation-coupled relationship; a fourth electroluminescent cell having one terminal connected to said third point, said fourth electroluminescent cell and said first photoconductor being positioned in radiation-coupled relationship, a third photoconductor connected in electrical series relationship with said thir electroluminescent cell and positioned in radiation-coupled relationship with said second electroluminescent cell, a fourth photoconductor connected in electrical series relationship wtih said fourth electroluminescent cell and positioned in radiation-coupled relationship with said first electroluminescent cell, means for connecting said first point to a first source of potential, means for connecting said third point to a second source of potential, and a switching means for momentarily connecting said third and fourth photoconductors to said first point through a low impedance path so that the series circuit consisting of said third electroluminescent cell and said third photoconductor and the series circuit consisting of said fourth electroluminescent cell and said fourth photoconductor are simultaneously placed in parallel between said first and thirdpoints.

2. A binary counter network as in claim 1 wherein said switching means comprises a fifth photoconductor having one terminal connected to said third and fourth photoconductors and a second terminal connected to said.

first point and a triggering means positioned in radiationcoupled relationship with said fifth photoconductor for applying a radiation input trigger signal thereto.

3. A binary counter network as in claim 1 wherein said switching means comprises a fifth photoconductor having one terminal connected to said third photoconductor and a second terminal connected to said first point, a second photoconductor having one terminal connected to said fourth photoconductor and a second'terminal connected to said first point, and a triggering means positioned in radiation-coupled relationship with said fifth and sixth photoconductorsfor simultaneously applying a 0 radiation input trigger signal to said fifth and sixth photoconductors.

4. A binary counter network comprising at least four electrically distinct connection points, a first electroluminescent cell and a first photoconductor connected in parallel between the first and second of said points, a second electroluminescent cell and a second photoconductor connected in parallel between the second and third of said points, the first electroluminescent. cell and the second photoconductor being further positioned in radiation-coupled relationshipythe second electroluminescent cell and the first photoconductor being further positioned in radiation-coupled relationship, a third electroluminescent cell and a third photoconductor connected'in series between the first and fourth of said points, a fourth electroluminescent cell and a fourth photoconductor connected in series between said first and fourth points, the third electroluminescent cell and the first photoconductor being further positioned in radiation-coupled relationship, the fourth electroluminescent cell; and the second photoconductor being further positioned in radiationcoupled relationship, the first electroluminescent cell being further positioned in radiation-coupled relationship with the third photoconductor, the second electroluminescent cell being further positioned in radiation-coupled relationship with the fourth photoconductor, a fifth photoconductor connected between the third and fourth of said points, a source of steady electrical energy, and means for connecting said source to said first and third points.

5. A network element as defined in claim 4 further including triggering means positioned in radiation-coupled relationship with said fifth photoconductor and adapted to apply a radiation input trigger signal thereto.

6. A network element as defined in claim 4 wherein the response time of saidthird and fourth photoconductors is substantially greater than the time necessary for said network element to switch from one of its two stable states to its otherstable state.

References Cited in the .file of this patent UNITED STATES PATENTS Loebner July 14, 1959 Loebner Sept; 29, 1959 

